Aberrant control of the initiation of cellular DNA replication leads to cancer.The purpose of the studies proposed in this application is to gain insight into the molecular mechanisms governing the initiation of DNA replication in mammalian cells, with the expectation that this knowledge will provide the means to reduce proliferation of neoplastic cells. The small DNA tumor viruses provide excellent experimental models with which to study DNA replication in mammalian cells. These viruses depend largely upon cellular machinery to replicate their genomes, and now much of the biochemistry of the replication complex that is assembled with viral and cellular proteins is understood. However, it is not understood how the assembly of this complex and its subsequent activity is regulated, and this knowledge should provide insights into how cellular replication is regulated. In these studies Dr. Folk proposes to utilize in vitro and in vivo replication assays with purified proteins and DNAs to focus upon two aspects of the regulation of DNA replication of the murine polyomaviruses: 1) The role of the viral tumor antigens in modifying cellular proteins and cellular signaling pathways to render the host cell permissive for DNA replication. In particular, the importance of a novel viral protein containing sequences capable of interacting with DNA polymerase-DNA primase will be explored. 2) The role of cellular proteins which bind the polyomavirus enhancer in activating the viral origin and the replication complex assembled thereupon. In particular, Dr. Folk will focus upon the AP1 protein, which is a paradigm for cellular transcription factors, and ets family proteins binding to the PEA3 motif.